1. Field of the Invention
The present invention relates to a liquid crystal display with enhanced response speed, transmittance, and aperture ratio and a method for manufacturing the same.
2. Description of the Related Art
Liquid crystal displays are widely used in television and graphic displays. Especially, since active matrix liquid crystal displays has high response speed and are compatible in display devices with many pixels, they greatly contribute to realizing high definition and larger sized displays.
These liquid crystal display devices comprise a pair of transparent glass substrates, thin film transistors and pixel electrodes formed on one of the glass substrates, and color filters, and counter electrodes formed on the other glass substrate. Liquid crystal material is injected and sealed inside the space between two glass substrates.
Twisted nematic mode liquid crystals are widely in use recently because of their superior optical characteristics. The liquid crystal molecules in the twisted nematic mode are parallel to the upper and lower glass substrates when no voltage is introduced to the same. Also, the molecules are continuously arranged so that a twisted angle between the upper most molecules and lower most molecules has 90.degree.. When voltage is introduced to the glass substrates facing each other, an electric field perpendicular to the glass substrates is formed and the liquid crystal molecules are arranged along the electric field. However, since the liquid crystal molecules of the twisted nematic(hereinafter abbreviated as TN) liquid crystal displays can be arranged only in one direction, a viewing angle is narrow.
Various technologies to enhance the viewing angle have been suggested. One of them is an IPS(In-Plane Switching) mode liquid crystal display. In the IPS mode liquid crystal display, counter electrodes usually formed on the upper substrate are formed on the lower glass substrate where pixel electrodes are formed. Hence, an electric field parallel to the substrates is formed between the pixel electrode and the counter electrode.
Referring to FIG. 1 and FIG. 2, the IPS mode liquid crystal display device comprises a gate line 2A in a column direction formed on a surface of a lower substrate 1 for scan image information to the upper part of the lower substrate; a counter electrode 2B on the same plane where the gate line 2A is, separated from the gate Line 2A by a predetermined distance; a gate insulation film 3 formed on a surface of the resultant structure where the gate line 2A and the counter electrode 2B are formed; a data line 5A which defines a pixel domain together with the gate line 2A, formed in the column direction and perpendicular to the gate line 2A that is separated by the gate insulation film 3; and a pixel electrode 5B lying on the same plane as the data line 5A over the counter electrode 2B, overlapping with some parts of the counter electrode 2B. The counter electrode 2B comprises a first portion 2BA and a second portion 2BC which are parallel to the gate line 2A; a third portion 2BB arranged perpendicularly to the first and second portion 2BA, 2BC, formed between a middle part of the first portion 2BA and a middle part of the second portion 2BC. The pixel electrode 5B has a first portion 5BA and a second portion 5BB which both have identical length and parallel to the gate line 2A; third and fourth portions 5BC, 5BD, both parallel to the data line 5A, respectively connected to the end parts of the first portion 5BA and the second portion 5BB; and a fifth portion 5BE which is protruded from the third portion 5BC, parallel to the data line 2A.
The first and second portions 2BA, 2BC of the counter electrode 2A, which overlaps with the pixel electrode 5B, are employed as an electrode of a capacitor for maintaining the liquid crystal molecules in a certain form for a predetermined period. One end of the fifth portion 5BE of the pixel electrode 5A where the pixel electrode 5B and the gate line 2A intersect, is employed as a drain for a thin film transistor. Furthermore, the semiconductor layer 4 formed at a predetermined part of the top surface of the gate insulation film 3 serves as a channel for a thin film transistor. One end of the data line 5A intersecting with the semiconductor layer 4 formed on the gate line 2A is employed as a source for a thin film transistor.
The above described gate line 2A and the counter electrode 2B is made of one selected from a group consisting of aluminum, titanium, tantalum, and chrome and combination thereof. The gate insulation film 3 is made by oxidizing one of the above metals or forming double insulation layer. The data line and the pixel electrode are formed by depositing one of the above mentioned metals and patterning the same.
As it is shown in the sectional view of FIG. 2, an electric field F1 parallel to the glass substrate is formed between the pixel electrode 5B and the counter electrode 2B. However, since the pixel electrode and the counter electrode are rectangular form, the edge effect causes a parabolic electric field F2 at the edges of these electrodes. The liquid crystal molecules are arranged along the electric field. Hence the longer axis of the molecules along the electric field F1 is parallel to the glass substrates. However, since the longer axis of the liquid crystal molecules has a slanting angle to the glass substrates due to the electric field F2, the liquid crystal molecules situated at the top surface of the electrodes 5BC, 2BC, 5BB which have longer axis perpendicular to the glass substrates, do not react to the introduced voltage. Thus, the response speed and the transmittance are inherently low.
The transmittance of a conventional liquid crystal display where the distance between the pixel electrode and the counter electrode is 210 .mu.m, the width of the pixel electrode or the counter electrode is 20 .mu.m, the liquid crystal molecules have a slanting angle of 22.degree. to the horizontal axis, and the introduced voltage to the pixel electrode is 8 V, is shown in FIG. 3, for example of the above description. In FIG. 3, the X stands for transmittance to the voltage introduced to the lower substrate, and the Y stands for a height of the liquid crystal layer from the lower substrate with the pixel electrode and the counter electrode. The transmittance after 100 ms from introducing voltage is about 38% and that before 100 ms is 30% or less.
Also, as the pixel electrode and the counter electrode are made of non-transparent metal, the transmittance is close to 0% in these areas. Hence, the aperture ratio of the liquid crystal display is reduced.